Aluminum nitride (AlN) is material that has a number of characteristics (i.e., structural, chemical, thermal and electrical) that make it an ideal candidate for a variety of applications including, but not limited to, sensors, light emitting diodes (LEDs), laser diodes (LDs) and insulating substrates for high frequency, high power electronics. AlN shares the same wurtzite crystal structure as GaN, thus epitaxial growth on AlN is not limited to the c-plane, but can also utilize the a- and m-planes. As such the polarization effects that are always present in films grown on c-plane substrates can be avoided by depositing the epitaxial layers on either the a- or m-plane. Additionally, the thermal conductivity of AlN is much higher than sapphire and comparable to that of 6H—SiC. Furthermore, AlN is chemically stable under AlxGa1-xN epitaxial growth conditions, thus allowing uncontaminated layers to be grown. One of this material's most interesting characteristics is its surface acoustic wave (SAW) velocity which is the highest ever reported. As such, it is an excellent candidate for both piezoelectric and SAW devices.
The inability to realize all of the benefits offered by AlN is largely due to the unavailability of bulk single crystals with a diameter of at least 2 inches. The most commonly used method to produce AlN single crystals is the sublimation method which utilizes vapor-phase crystallization of an evaporated solid source. The primary difficulty encountered during the growth of AlN results from the strong reaction between the crucible material and the AlN vapors at high temperatures. This problem is exacerbated due to long growth cycles such as those required to grow large crystals, and due to the desired high growth temperatures.
High growth temperatures, for example temperatures in excess of 2200° C., provide the higher growth rates that are desirable for the growth of large, bulk AlN crystals. Additionally, the use of high growth temperatures helps reduce the thermal stress in the AlN crystals since such temperatures permit the use of smaller temperature gradients. By reducing thermal stress in the growing crystal, crystalline defects can be minimized. High growth temperatures also allow the aluminum and nitrogen atoms to locate in the best equilibrium lattice positions since surface adatom mobilities increase with temperature.
The most common refractory material used to grow AlN is graphite. It is relatively inexpensive and easy to mechanically process. Due to the electrical properties of graphite, it can be used in growth systems utilizing either resistance or RF heating. Unfortunately graphite does not have sufficient thermal stability to be used at temperatures greater than 1000° C. Furthermore, as a result of graphite's thermal instability, graphite crucibles degrade rapidly, often resulting in changes in the heat field distribution within the growth cell and unstable growth parameters. To counter this effect, growth cycles may be conducted in an inert atmosphere (e.g., argon, helium). However even under these growth conditions there are sufficient aluminum and nitrogen vapors to react with the graphite, leading to the graphite crucible's deterioration and ultimately its failure. Another disadvantage of graphite is that even the purest grades of graphite exhibit high impurity concentrations (e.g., boron, aluminum, nickel, chromium, copper, etc.) that affect the electrical properties and overall quality of the grown crystal.
Tantalum carbide (TaC) is another material that researchers have tried to use to grow AlN crystals. TaC crucibles have been used quite favorably to grow silicon carbide (SiC) crystals, in part because carbon is a constituent of both SiC and the crucible. If a TaC crucible is used to grow AlN crystals, however, the nitrogen vapors formed by the evaporating AlN source interact with the TaC crucible, resulting in nitrogen substituting for the carbon in the crucible and the vapor phase becoming doped with carbon. As the substitution process is most intense during the initial stages of growth, and as the initial stages of growth define the quality of the growing crystal, it is virtually impossible to grow a high quality AlN crystal with a TaC crucible.
Accordingly, what is needed in the art is a method that allows high quality, large diameter AlN single crystals to be grown. The present invention provides a crucible suitable for growing such crystals as well a method of manufacturing the same.